1. Field of the Invention
The present invention relates to charge pumps used for providing high voltages to digital memories, and more particularly, to charge pumps which operate on low voltage power supplies.
2. Description of the Related Art
Many non-volatile digital memories, such as EEPROMs, EPROMs, and FLASH memories, require relatively high voltage potentials in order to write or erase information. Typically, the required voltage is approximately 12-20 Volts. Because the power supplies of most digital systems supply a much lower voltage, such as 2-5 Volts, various methods of high-voltage generation have been used to "pump up" the supply voltage to the potential required by non-volatile memories.
FIG. 1 illustrates a conventional charge pump 20, a device, well known in the art, used for high-voltage generation. The charge pump 20, which is also referred to as a voltage multiplier, increases (or "pumps up") the amplitude of the voltage supply signal Vcc to the programming/erase voltage Vpp required by non-volatile memories.
The charge pump 20 is made up of several series-connected pump stages 22, 24, 26, and 28. Pump stage 28 represents the nth pump stage. Each pump stage 22, 24, 26, and 28 is respectively made up of capacitors C2, C3, C4, and Cn connected to the respective anodes of diodes D2, D3, D4, and Dn. The voltage supply signal Vcc is coupled through a diode D1 to the anode of diode D2 of pump stage 22. The pumped up voltage signal Vpp is received from the cathode of diode Dn of pump stage 28. Each pump stage 22, 24, 26, and 28 is coupled to one of a pair of complementary clock signals .PHI..sub.1 and .PHI..sub.2 through capacitors C2, C3, C4, and Cn, respectively.
During operation, the capacitors C2, C3, C4, and Cn are successively charged and discharged during each half of the clock signal. Specifically, capacitor C2 is charged through diode D1 as clock signal .PHI..sub.1 goes low; capacitor C4 is charged via the path through capacitor C3, diode D3 and positive-going clock .PHI..sub.2 at the same time. Charge is transferred from capacitor C2 to capcitor C3 through diode D2 when the clocks reverse polarity. Charge transfer is constrained by the diodes to be from left to right.
As the capacitors C2, C3, C4, and Cn are successively charged and discharged, packets of charge are "pumped" along the diode chain D2, D3, D4, and Dn. The average voltage potential at the cathodes of diodes D2, D3, D4, and Dn increases progressively from the input to the output of the diode chain. Thus, the voltage signal generated at the cathode of diode Dn has a greater amplitude than the voltage supply signal Vcc.
When the voltage supply signal is 5 Volts, the charge pump 20 is sufficient for generating the high voltages required by non-volatile memories. In this scenario, approximately 10 pump stages are utilized. Furthermore, it is common for diodes D2, D3, D4, and Dn, and capacitors C2, C3, C4, and Cn, to be implemented with diode-connected and capacitor-connected MOS transistors.
Many of today's digital systems, however, such as portable computers, have power supplies that supply voltages less than 5 Volts. Charge pumps designed for 5 Volt supplies are inadequate for low voltage power supplies for two reasons. First, additional pump stages are required because the incremental voltage increase per stage is small. As the number of pump stages increases, the effective resistance of the charge pump increases, which limits the amount of current that can be supplied to the non-volatile memories. Second, a phenomenon known as "body effect" (or "M factor") causes the effective threshold voltages of the capacitor-connected transistors in the upper pump stages to increase. Because the clock signals have small amplitudes, body effect causes the transistor conductance to decrease. As conduction of the transistors decreases, the upper pump stages become highly resistive, which severely limits current.
FIG. 2 illustrates a prior art attempt to solve the inadequacies of conventional charge pumps when used with low voltage power supplies. A first-stage charge pump 30, which has the same general design as charge pump 20, is powered by a 2 Volt power supply signal Vcc. A first oscillator 32, which is also powered by Vcc, generates a pair of complementary clock signals .PHI..sub.1 and .PHI..sub.2 for use by first-stage charge pump 30. The output Vo of first-stage charge pump 30 is used as the power supply signal for a second oscillator 34. The second oscillator 34 generates a pair of complementary clock signals .PHI..sub.1.sup.+ and .PHI..sub.2.sup.+ for use by second-stage charge pump 36. The second stage charge pump 36, which also has the same general design as charge pump 20, is powered by voltage supply signal Vcc.
During operation, first stage charge pump 30 increases ("pumps up") the amplitude of voltage supply signal Vcc, resulting in output signal Vo. Because output signal Vo is used as a power supply signal for second oscillator 34, complementary clock signals .PHI..sub.1.sup.+ and .PHI..sub.2.sup.+ have greater amplitudes than clock signals .PHI..sub.1 and .PHI..sub.2. Although second stage charge pump 36 is powered by low voltage supply signal Vcc, the clock signals .PHI..sub.1.sup.+ and .PHI..sub.2.sup.+, which have increased amplitudes, cause second stage charge pump 36 to increase ("pump up") the voltage supply signal Vcc more than usual. Thus, a programming/erase voltage signal Vpp in the 12-20 Volt range is produced.
The two-stage charge pump of FIG. 2 suffers from a number of disadvantages. First, two oscillators are required which increases the physical size of the pump and consumes additional power. Second, very large elements are required in the first-stage pump 30 in order to provide sufficient power to the second oscillator 34. The larger elements require more power from the first oscillator 32 which cause it to consume substantial quantities of power. Energy efficiency is very important in low voltage supply applications.
Thus, there is a need for a charge pump that can generate the high voltages required by non-volatile memories from a low voltage power supply without being unduly resistive, power consuming, or physically large.